Traumatic battlefield wounds leading to limb amputation have profound morbidity for members of the armed forces. There were 6,144 cases of traumatic amputations in 5,694 service personnel from 2000 to 2011 according to Medical Surveillance Monthly Reports (MSMR) published by the Armed Forces Health Surveillance Center (AFHSC). One of the key and efficacious treatments of limb prosthetics is to attach the prosthetic limb to a bone anchor protruding from an amputee stump. Direct bone anchorage means that the prosthesis is attached without using a socket fitted over the stump of the amputated limb. The method is based on the principle of osseointegration, which has been in clinical use for prosthetic replacement of teeth since 1965. For example, by surgically implanting a titanium screw, known as a fixture, into the femur (thigh bone) produces a direct attachment for a prosthetic leg. Osseointegration refers to the fusion of the implant surface with the surrounding bone. The concept of osseointegration entails a direct contact between the fixture and the bone tissue, thereby assuring a stable attachment.
However, upper limb (arm, shoulder) reconstruction is more challenging due to the lower loading and potentially reduced bone healing capability as described by Wolff's law. Wolff's law states that bone in a healthy person or animal will adapt to the loads under which the bone is placed. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading. The internal architecture of the trabeculae undergoes adaptive changes, followed by secondary changes to the external cortical portion of a bone, perhaps becoming thicker as a result. Conversely, if the loading on a bone decreases, the bone will become weaker due to turnover, and it is less metabolically costly to maintain and there is no stimulus for continued remodeling that is required to maintain bone mass. Of the US military amputations detailed above from the years 2000 to 2011, 3,339 were upper extremity amputations, 500 which were major composed of 32 amputations that occurred at the hand/wrist, 223 of the forearm or below the elbow, 216 at or above the elbow, and 29 bilateral. Upper extremity bones are generally subjected to lower loading than the weight bearing bones found in the lower extremities or legs.
Bone anchored limb prostheses require chronic through-the-skin (percutaneous) attachment without infection. Infection can result in loosening and detachment but also serious morbidity and mortality from sepsis. Various approaches to mitigating infection have included bound antimicrobials.
A typical treatment for leg prosthesis involves two surgical procedures. In the first operation, a fixture titanium screw is inserted into the residual femur of the remaining portion of the limb. The fixture is then allowed to heal into the bone for 6 months with no load. During this period it is usually possible to use a standard socket prosthesis as soon as the residual limb has healed. In the second procedure an extension known as an abutment is attached to the bottom of the fixture. The abutment protrudes from the skin penetration area and serves as the attachment onto which the prosthesis is affixed. FIG. 1A is a photograph of an abutment and skin penetration area with a fixture for direct attachment for a prosthetic leg. In order to ensure a solid attachment between the titanium fixture and the bone, the bone needs to be carefully subjected to a load before the patient can start walking properly with prosthesis again. For leg replacement loading on the bone and fixture is accomplished through controlled, gradually increased training using a short ‘training prosthesis’ as shown in FIG. 1B. It is not possible to walk with the training prosthesis. The real prosthesis is tested around 3 months after the second operation. In the months that follow the prosthesis can gradually be used more and more, although always with a pair of crutches. Walking without support or with only one crutch is possible around 6 months after the second operation. The first operation generally requires 5-7 days in hospital, and the second about 10 days. The overall length of the treatment for fixture attachment for a limb prosthetic including the two operations, rehabilitation and prosthesis provision is estimated at around 12 months for patients with a normal bone quality.
Bone anchors are also widely used in the field of dentistry in the form of dental implants. A dental implant is a “root” device, typically made of titanium, used in dentistry to support restorations that resemble a tooth or group of teeth to replace missing teeth. Dental implants are generally root-form endosseous implants, and are placed within the bone. The bone of the jaw or skull accepts and osseointegrates with the titanium post. Dental implants fuse with bone; however, the implants lack the periodontal ligament, so they feel slightly different from natural teeth during chewing. Failure of a dental implant is often related to the failure of the implant to osseointegrate correctly with the bone, or vice-versa. A dental implant is considered to be a failure if it is lost, mobile or shows peri-implant (around the implant) bone loss of greater than 1.0 mm in the first year and greater than 0.2 mm a year after.
Despite the advances in prosthetic attachment to replace missing portions of limbs and dental implantation through bone anchors, there exists a need for improved methods and therapies to enhance and hasten healing, and bone anchor function in less than an ideal load and wound environments with non-optimal healing (e.g., Heterotopic ossification, limited soft tissue and skin) and non-optimal limb length for prosthetics.